1. Field of the Invention
The present invention relates to a radiographing system, a method of controlling automatic exposure in a radiographing system, and a radiological image detection device.
2. Description of the Related Art
In the field of medicine, an X-ray radiographing system using radiation, for example, X-rays is known. The X-ray radiographing system has an X-ray generation device which generates X-rays, and the X-ray radiographing device which receives the X-rays to radiograph an X-ray image. The X-ray generation device has an X-ray source which irradiates the X-rays toward a subject, a source control device which controls the driving of the X-ray source, and an irradiation switch which is used to input an X-ray irradiation start instruction. The X-ray radiographing device has an X-ray image detection device which receives the X-rays transmitted through the subject to detect the X-ray image, and a console which controls the driving of the X-ray image detection device and performs various kinds of image processing on the X-ray image.
In recent years, in the field of X-ray radiographing systems, an X-ray image detection device in which a flat panel detector (FPD) is used as a detection panel instead of X-ray film or an imaging plate (IP) has come into in widespread use. In the FPD, pixels which accumulate signal charges depending on the amount of incident X-rays are arranged in a matrix. The FPD stores signal charges in each pixel, converts the accumulated signal charges to a voltage signal to detect an X-ray image representing image information of the subject, and outputs the X-ray image as digital image data.
An electronic cassette (portable X-ray image detection device) in which an FPD is embedded in a rectangular parallelepiped housing has also been put into practical use. Unlike a type which is fixed to a radiography platform and is not detachable, the electronic cassette is used in a state of being detachably attached to an existing radiography platform for a film cassette or an IP cassette, or a dedicated radiography platform, or placed on a bed or attached to the subject so as to radiograph a region which is not easily radiographed using a fixed type. In order to radiograph an old person who is recuperating at home or an emergency patient caused by an accident or a disaster, the electronic cassette may be used at place with no radiography platform outside a hospital.
In the X-ray radiographing system, a sensor, such as an ionization chamber, which detects the X-ray dose transmitted through the subject is provided, and after the accumulated value of the X-ray dose detected by the sensor reaches a threshold value set in advance, automatic exposure control (AEC) to prohibit the irradiation of the X-rays from the X-ray source is performed.
In JP2008-220724A, an exposure prohibition threshold value (irradiation prohibition threshold value) of AEC changes depending on X-ray transmittance or a structure, a difference in material, or the type of cassette, such as the type of scintillator.
JP2003-302716A describes that a phototimer (AEC sensor) is embedded in an electronic cassette, and radiographing is possible at a plurality of X-ray tubes (X-ray sources) with one phototimer. An output signal from the phototimer may be either an X-ray cutoff signal (irradiation prohibition signal) or an analog signal (detection signal, voltage value). In the former case, the charges from the phototimer are integrated (accumulated) inside the electronic cassette, and the integrated value (accumulated value) is compared with a threshold value, when the integrated value exceeds the threshold value, the X-ray cutoff signal is output. In the latter case, the analog signal is integrated on the reception side (X-ray generation device side) and the X-rays are cut off through comparison with the threshold value.
JP2004-166724A describes an AEC pixel-embedded radiation detector in which the detection value (detection signal) of the AEC pixel is calibrated depending on the grid attachment/detachment state, the grid type, the radiographing condition, or the like. An exposure prohibition signal (irradiation prohibition signal) is output to an X-ray detection device depending on the comparison result of the calibrated detection value and an exposure prohibition threshold value (irradiation prohibition threshold value).
JP-1995-201490A (JP-1995-201490A (JP-H07-201490A)) describes that a predetermined pixel from among a plurality of pixels is used as an X-ray exposure amount detection pixel (AEC sensor).